Nozzle bodies for modern direct injection systems are used to an increasing extent at operating temperatures up to 450° C. High demands are therefore made on the strength of components and the wear resistance of nozzle bodies. Nitrided hot forming tool steel in particular is therefore used to manufacture the nozzle bodies. ECM (electrochemical machining) methods are used in the production of internal bores (pressure chambers) and for rounding. The ECM methods used for shaping and surface treatment of metal workpieces are performed in an electrolyte solution, the workpiece to be machined usually being connected as the anode and the tool being connected as the cathode. Electrochemical machining methods are used in particular for deburring, polishing, grinding and etching the surfaces of a workpiece. The workpieces formed by the ECM method are highly passive and are very difficult to treat by thermochemical diffusion methods, in particular nitriding, because more noble alloy elements such as Cr remain on the surface and/or oxide alloy elements become oxidized, forming metal oxides and metal hydroxides MexOy[OH]z.
To improve the nitridability of direct-injection nozzle bodies, it is conventional today to pickle passive surfaces before nitriding, in particular by using hydrochloric acid. However, pickling has some major disadvantages. Pickling with acid may cause pickling scars, which decrease the strength of the component. Furthermore, it is very difficult to reproduce the results of pickling, because the length of storage between machining, basic heat treatment and nitriding may vary. Furthermore, pickling results in a considerable additional cost which is attributable in particular to the cost of the installation used for pickling and the required labor cost. Pickled workpieces must also be cleaned after pickling by using a very complex special cleaning technique. Disposal of pickling solutions is also complicated. In addition, pickling with acid results in unwanted environmental pollution and has a negative effect on working conditions.
The object of the present invention is thus to develop a method of treating workpieces made of hot forming tool steel, in particular direct injection nozzle bodies, to improve the nitridability of these workpieces in particular without having to pickle the workpieces and to thus avoid the disadvantages due to pickling which are known in the related art.